Environmental Chemistry

Pollution is the unwanted change in our environment, and the substance that causes it is called a pollutant.

• A pollutant is the substance that causes pollution, e.g. CO, SO₂, smoke, sewage.
• Biodegradable pollutants decompose naturally — e.g. sewage, food waste, paper.
• Non-biodegradable pollutants do not decompose — e.g. plastics, DDT, glass, metals.
• Primary pollutants are emitted directly (e.g. CO, SO₂); secondary pollutants form by reaction in the air (e.g. ozone, PAN, acid rain).

Pollution is named after the part of the environment it affects.

• The safe limit of noise in a residential area is about 45–55 dB.

A handful of gases and particles do most of the damage to air quality, and the exam asks repeatedly about their sources and effects.

• PM₂.₅ (particles smaller than 2.5 micrometres) is the most harmful as it penetrates deep into the lungs.

Carbon monoxide is the silent killer of air pollutants because of the way it attacks the blood.

• Its main source is motor-vehicle exhaust.
• CO combines with haemoglobin to form carboxyhaemoglobin, which is far more stable than oxyhaemoglobin.
• This reduces the oxygen-carrying capacity of the blood, causing headache, dizziness and even death.

When the gases SO₂ and NO₂ mix with rainwater, the rain itself turns acidic and corrodes everything it falls on.

• These acids fall to the ground as acid rain.
• Normal rain is slightly acidic (pH ≈ 5.6) due to dissolved CO₂; rain is called acid rain when its pH falls below 5.6.

• Corrodes buildings and monuments made of marble (CaCO₃) — known as "Marble Cancer".
• The Taj Mahal is damaged by acid rain due to nearby industries (the Mathura refinery), its white marble yellowing and decaying.
• Lowers soil fertility and harms crops and forests.
• Makes lakes & rivers acidic, killing fish and aquatic life.
• Acid rain reaction: CaCO₃ + H₂SO₄ → CaSO₄ + H₂O + CO₂ — the marble dissolves away.

High above us a thin layer of ozone acts as Earth's sunscreen, soaking up the Sun's most harmful rays.

• Ozone (O₃) is a form of oxygen with three atoms.
• It absorbs the Sun's harmful ultraviolet (UV) rays, protecting life from skin cancer and eye damage.
• At ground level (troposphere), ozone is a pollutant, but in the stratosphere it is protective — "good up high, bad nearby."

The same chemicals that once cooled our refrigerators turned out to be tearing holes in this protective shield.

• In the stratosphere, UV light breaks CFCs to release chlorine atoms, which destroy ozone (one chlorine atom destroys thousands of O₃ molecules).
• The "ozone hole" is the thinning of the ozone layer over Antarctica, largest around September–October.
• More UV reaching the ground causes skin cancer, cataracts and weak immunity.
• 16 September is the International Day for the Preservation of the Ozone Layer.

Certain gases in the air trap the Sun's heat like the glass of a greenhouse, and too much of them is steadily warming the planet.

• An excess of greenhouse gases causes global warming — a rise in the Earth's average temperature.
• Global warming leads to melting glaciers, rising sea levels and climate change.
• CO₂ is the main greenhouse gas responsible for global warming.

• Methane (CH₄) is a more powerful greenhouse gas than CO₂ per molecule.

Smog is the dirty haze that hangs over big cities, and it comes in two distinct chemical forms.

• Classical (London / sulphurous) smog forms in cold, humid weather from SO₂ and smoke — it is reducing in nature.
• Photochemical (Los Angeles) smog forms in warm, sunny weather from NOₓ and hydrocarbons reacting in sunlight — it is oxidising in nature.
• Photochemical smog contains ozone (O₃) and PAN (peroxyacetyl nitrate), which irritate the eyes and lungs.
• Smog reduces visibility and worsens breathing problems.

Water looks clean long after it has stopped being safe, so scientists measure pollution by how much oxygen the water can no longer hold.

• High BOD = more pollution (more organic waste, less clean water).
• Clean water has a low BOD; heavily polluted/sewage water has a high BOD.

• Eutrophication is the excessive growth of algae in a water body due to nutrients like nitrates & phosphates from fertilisers.
• The algal bloom blocks sunlight and uses up dissolved oxygen, killing fish and aquatic life.
• Dissolved oxygen (DO) below ~4 mg/L is harmful to aquatic life.

A few specific contaminants in drinking water are repeatedly tested in the exam.

The world has signed several treaties to repair the ozone layer and slow global warming — and the exam loves matching each one to its purpose.

A short list of dates is almost guaranteed to appear in General Awareness.